Extend raid 5

Extend raid 5

[buliwyf]

Hello

I have the following problem.
I have set up a Linux Software Raid 5 with 4 160GB Disks.
This works well. Now i run out of Disk Space and want to add another
160 GB Disk on the fly. Is this possible without loosing Data?
my /etc/raidtab

raiddev /dev/md0
        raid-level      5
        nr-raid-disks   4
        nr-spare-disks  0
        persistent-superblock 1
        parity-algorithm        left-symmetric
        chunk-size      32
        device          /dev/hdi1
        raid-disk       0
        device          /dev/hdj1
        raid-disk       1
        device          /dev/hdk1
        raid-disk       2
        device          /dev/hdl1
        raid-disk       3

My system is a Debian/gnu Linux Woody Box.
Installed is Raidtools2 and mdadm.
I tried the raidreconf tool but always got an error message.
Has anybody the same problem yet and has solved it, or can tell me
a solution?

cYa
Dominik Sennfelder

Re: Extend raid 5

[Mike Fedyk]

On Mon, Jan 12, 2004 at 01:11:55AM +0100, buliwyf wrote:
> Hello
> 
> I have the following problem.
> I have set up a Linux Software Raid 5 with 4 160GB Disks.
> This works well. Now i run out of Disk Space and want to add another
> 160 GB Disk on the fly. Is this possible without loosing Data?

No, the raid format splits the raid stripes sequencially between the drives
available in the array.  So adding an extra online disk would really mess up
the formatting of the array, and therefore isn't allowed.

There really isn't a way to get the redundancy of raid and the size addition
capabilities of LVM when you only want to add one disk.

If you had lvm on top of raid, you could add that one disk to your lvm set,
but it wouldn't be redundant unless you added two disks and added a raid1
array to your lvm set.

I'd also suggest you get an extra drive as a spare for your raid5 array.

I'd suggest you add the drive as a seperate filesystem until you can setup
your array with lvm on raid.

Mike

Re: Extend raid 5

[Maarten v d Berg]

On Monday 12 January 2004 03:13, Mike Fedyk wrote:
> On Mon, Jan 12, 2004 at 01:11:55AM +0100, buliwyf wrote:

> > I have the following problem.
> > I have set up a Linux Software Raid 5 with 4 160GB Disks.
> > This works well. Now i run out of Disk Space and want to add another
> > 160 GB Disk on the fly. Is this possible without loosing Data?
>
> No, the raid format splits the raid stripes sequencially between the drives
> available in the array.  So adding an extra online disk would really mess
> up the formatting of the array, and therefore isn't allowed.
>
> There really isn't a way to get the redundancy of raid and the size
> addition capabilities of LVM when you only want to add one disk.

[...]
> I'd suggest you add the drive as a seperate filesystem until you can setup
> your array with lvm on raid.

Hi Mike.  I read about LVM some time ago and decided to use that as it seemed 
to solve my problem of the ever-growing data volume I need to store.

However, after the initial setup I quickly dropped the LVM idea again since it 
only _seemed_ to solve my problem. As I understand it, LVM allows the 
addition of extra volumes but it does nothing at the level of the filesystem 
which resides on top. So in order to effectively grow my filesystem, which is 
the ultimate goal of course, I'd need to delete the current FS and make a new 
-bigger- one. And last time I checked this definitely kills your data.

Otherwise, adding a 40 GB physical volume to a 120 GB raid5 / LVM set just 
gives me one 120 GB partition and [room for] another 40 GB partition. 
There is NO gain whatsoever using LVM here compared to when I would just have 
added a single 40GB disk all by itself without using LVM in the first place, 
is there ?

This has always left me wondering.  Did I miss something (except using some 
alpha FS-resize code...) ?

Greetings,
Maarten

Re: Extend raid 5

[Måns Rullgård]

Maarten v d Berg <maarten@vbvb.nl> writes:

> Hi Mike.  I read about LVM some time ago and decided to use that as
> it seemed to solve my problem of the ever-growing data volume I need
> to store.
>
> However, after the initial setup I quickly dropped the LVM idea
> again since it only _seemed_ to solve my problem. As I understand
> it, LVM allows the addition of extra volumes but it does nothing at
> the level of the filesystem which resides on top. So in order to
> effectively grow my filesystem, which is the ultimate goal of
> course, I'd need to delete the current FS and make a new -bigger-
> one. And last time I checked this definitely kills your data.
>
> Otherwise, adding a 40 GB physical volume to a 120 GB raid5 / LVM
> set just gives me one 120 GB partition and [room for] another 40 GB
> partition.  There is NO gain whatsoever using LVM here compared to
> when I would just have added a single 40GB disk all by itself
> without using LVM in the first place, is there ?
>
> This has always left me wondering.  Did I miss something (except
> using some alpha FS-resize code...) ?

Yes, ext2, ext3, xfs, jfs and reiserfs can all be extended without
destroying the data.  Some of them can be reduced, too.

-- 
Måns Rullgård
mru@kth.se

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Re: Extend raid 5

[Marc Bevand]

Maarten v d Berg wrote:
> [...]
> Hi Mike.  I read about LVM some time ago and decided to use that as it seemed 
> to solve my problem of the ever-growing data volume I need to store.
> 
> However, after the initial setup I quickly dropped the LVM idea again since it 
> only _seemed_ to solve my problem. As I understand it, LVM allows the 
> addition of extra volumes but it does nothing at the level of the filesystem 
> which resides on top. So in order to effectively grow my filesystem, which is 
> the ultimate goal of course, I'd need to delete the current FS and make a new 
> -bigger- one. And last time I checked this definitely kills your data.
> 
> Otherwise, adding a 40 GB physical volume to a 120 GB raid5 / LVM set just 
> gives me one 120 GB partition and [room for] another 40 GB partition. 
> There is NO gain whatsoever using LVM here compared to when I would just have 
> added a single 40GB disk all by itself without using LVM in the first place, 
> is there ?
> 
> This has always left me wondering.  Did I miss something (except using some 
> alpha FS-resize code...) ?

This is precisely the point, you have to resize your filesystem so that
the extra space added to your LVM device is used. There are many
options, you can either use a userland tool (resize2fs, resize_reiserfs,
...) for resizing an *unmounted* filesystem, or you can do it in the
kernel (mount -o remount,resize=<size> <device>). As you can see, doing
it in the kernel has the extra advantage of allowing you to resize a
*mounted* filesystem.

Filesystem resizing is more stable than you think, for example the
commercial program Partition Magic is based on resize2fs (but I am not
sure if I can convince you with this example since proprietary software
is evil :P).

-- 
Marc Bevand

Re: Extend raid 5

[Maarten v d Berg]

On Monday 12 January 2004 11:02, Marc Bevand wrote:
> Maarten v d Berg wrote:
> > [...]

> > Otherwise, adding a 40 GB physical volume to a 120 GB raid5 / LVM set
> > just gives me one 120 GB partition and [room for] another 40 GB
> > partition. There is NO gain whatsoever using LVM here compared to when I
> > would just have added a single 40GB disk all by itself without using LVM
> > in the first place, is there ?
> >
> > This has always left me wondering.  Did I miss something (except using
> > some alpha FS-resize code...) ?
>
> This is precisely the point, you have to resize your filesystem so that
> the extra space added to your LVM device is used. There are many
> options, you can either use a userland tool (resize2fs, resize_reiserfs,
> ...) for resizing an *unmounted* filesystem, or you can do it in the
> kernel (mount -o remount,resize=<size> <device>). As you can see, doing
> it in the kernel has the extra advantage of allowing you to resize a
> *mounted* filesystem.

My raid filesystem is not part of the normal linux FS tree, so for me it is 
perfectly okay to umount the system.  I tend to only use reiserfs.

> Filesystem resizing is more stable than you think, for example the
> commercial program Partition Magic is based on resize2fs (but I am not
> sure if I can convince you with this example since proprietary software
> is evil :P).

I know partition magic but at the time I tried it it did not understand 
reiserfs so I dropped it. I don't know what the current version can do.

In any case I didn't want to run the risk at the time; I had something which 
could be called a "backup" (with some imagination) but it consisted of 
several tapes, disks and whatnot that could help restoring in case of a 
disaster but it was by no means near anything complete nor recent.
In other words, restoring would have cost me at least a full weekend and would 
have cost me anything between 5 - 20% of my data. I can accept that kind of 
risk for statistic 'normal' disasters but not for experiments with a higher- 
than-normal risk of losing the entire filesystem. 

The problem with adding a non-redundant drive to an existing raid-based LVM 
persists however. By adding that one drive and extending the FS to include 
that you introduce a single point of failure. Bye bye raid-redundancy...

Greetings,
Maarten

RE: Extend raid 5

[Guy]

LVM
You can extend a logical volume with "lvextend".
You can also extend a filesystem, not sure if this is safe.
Don't add a single disk.  Create a RAID1 or RAID5, and add that.  It may
cost more, but can you risk data loss?
I think I have read that there is a utility to add a disk to a RAID5.  But I
think it is not considered safe/stable.  I can't find any info about it.


-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Maarten v d Berg
Sent: Monday, January 12, 2004 4:21 AM
To: linux-raid@vger.kernel.org
Subject: Re: Extend raid 5

On Monday 12 January 2004 03:13, Mike Fedyk wrote:
> On Mon, Jan 12, 2004 at 01:11:55AM +0100, buliwyf wrote:

> > I have the following problem.
> > I have set up a Linux Software Raid 5 with 4 160GB Disks.
> > This works well. Now i run out of Disk Space and want to add another
> > 160 GB Disk on the fly. Is this possible without loosing Data?
>
> No, the raid format splits the raid stripes sequencially between the
drives
> available in the array.  So adding an extra online disk would really mess
> up the formatting of the array, and therefore isn't allowed.
>
> There really isn't a way to get the redundancy of raid and the size
> addition capabilities of LVM when you only want to add one disk.

[...]
> I'd suggest you add the drive as a seperate filesystem until you can setup
> your array with lvm on raid.

Hi Mike.  I read about LVM some time ago and decided to use that as it
seemed 
to solve my problem of the ever-growing data volume I need to store.

However, after the initial setup I quickly dropped the LVM idea again since
it 
only _seemed_ to solve my problem. As I understand it, LVM allows the 
addition of extra volumes but it does nothing at the level of the filesystem

which resides on top. So in order to effectively grow my filesystem, which
is 
the ultimate goal of course, I'd need to delete the current FS and make a
new 
-bigger- one. And last time I checked this definitely kills your data.

Otherwise, adding a 40 GB physical volume to a 120 GB raid5 / LVM set just 
gives me one 120 GB partition and [room for] another 40 GB partition. 
There is NO gain whatsoever using LVM here compared to when I would just
have 
added a single 40GB disk all by itself without using LVM in the first place,

is there ?

This has always left me wondering.  Did I miss something (except using some 
alpha FS-resize code...) ?

Greetings,
Maarten

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